1. The Field of the Invention
The present invention generally relates to statistical disbribution analysers and, more particularly, to analysers which measure the distributions of discrete levels of signals on a logarithmic scale, such as decibels.
2. Description of the Prior Art
Many times in the study of continuous variables, it is desired to know the statistical distribution of the measured variable. For example, in acoustics one desires to know the precent time that the sound pressure level is above various levels. In engineering problems where the unit of measure of the variable is in a logarithmic scale, such as decibels, classical statistical distribution analysers employ some form of a logarithmic amplifier followed by some form of analog to digital conversion. However, these types of circuits are expensive due to the high cost of logarithmic amplifiers and are only as accurate as the amplifier.
A standard type of analog to digital converter, such as the one disclosed by U.S. Pat. No. 3,613,111, converts incoming analog voltages to binary notation. The digital output line for any one voltage consists of an output at every level which is equal to or below the highest level of binary notation. Actually, each output line is actually the "OR"ing of the digital output with all of the other output lines which are more significant.
By definition, the decibel is one-tenth of the logarithm of the ratio of the measured sound power and a reference sound power, or EQU n = 10 log (W/W.sub.o) (1)
Where EQU n = decibels EQU W = sound power (watts) EQU W.sub.o == reference sound power (watts)
When the conditions are such that scalar ratios of currents or voltages (or analogous quantities in other fields such as pressures, amplitudes, or particle velocities in sound) are the square roots of the corresponding power ratios, the number of decibels by which the corresponding power ratios differ is expressed by the following formulae: EQU n = 20 log (V.sub.1 /V.sub.2) (2) EQU n = 20 log (I.sub.1 /I.sub.2) (3)
where I.sub.1 /I.sub.2 and V.sub.1 /V.sub.2 are the given current and voltage ratios, respectively.